The present disclosure relates to seismic surveying, for example, for the exploration and production of petroleum reservoirs, and more specifically to the optimal use of seismic sources comprised of forces on the surface of the earth. Such seismic sources may include vibrators to be used in the vibroseis (swept frequency vibrator source) technique. Such seismic surveys may include, without limitation, 2D, 3D, time-lapse 4D, permanent reservoir monitoring, active hydrofracture monitoring, and vertical seismic surveys.
It has long been recognized that the use of active seismic sources for various types of seismic surveying may often involve significant amounts of deleterious source-generated-noise. This noise may include various modes of propagation associated with the surface or near surface of the earth such as refractions, Rayleigh waves, Love waves, and air-blast. The term ground-roll is often generically used to describe source generated noise associated with the surface and near surface of the earth. Ground-roll is often understood to include significant amounts of particularly Rayleigh waves.
There is a long term trend in seismic surveying for oil and gas exploration and production to try to improve signals, such as reflected body waves or other desired seismic energy, vs. noise, particularly source generated noise such as Rayleigh waves. There is a continuing need for economical and practical means to reduce this interfering source generated noise.
Those skilled in the art will recognize that elastic wave concepts often yield a more realistic mathematical and physical model of the actual subsurface structure of the earth's subsurface than simpler acoustic models. See, for example Aki and Richards (2002). Concepts such as attenuation and non-linearities are also recognized as often being important in modeling the structure and formation characteristics of the earth's subsurface
Those skilled in the art will recognize that the use of the ground force concept has been widely used in Vibroseis seismic surveys for several decades. It is recognized that ground force concepts do not typically consider any aspects of the interaction between multiple seismic vibrator units. See, for example, U.S. Pat. No. 4,184,144 (Rickenbacker), Sallas (1984), and Sallas and Weber (1982).
Those skilled in the art will also recognize that there have been technical developments in Vibroseis technology to improve the tow frequency output of seismic vibrators. This technology includes displacement limits on vibrators, flow limits on vibrators, and non-linear sweep rates to emphasis low frequencies. See, for example, U.S. Pat. No. 7,327,633 B2 (“Systems and Methods for Enhancing Low-Frequency Content in Vibroseis Acquisition”, C. Bagaini, et. al., Feb. 5, 2008) and U.S. Pat. No. 8,275,862 B2 (“System and Method for Determining A Frequency Sweep for Seismic Analysis”, J. Sallas, Sep. 25, 2012). It is recognized that these technologies and concepts do not typically consider any aspects of the interaction between multiple seismic vibrator units.
The use of simultaneous vibrator seismic sources has been long recognized. See, for example, U.S. Pat. No. 4,823,326 (“Seismic Data Acquisition Technique Having Superposed Signals”, R. M. Ward, Apr. 18, 1989). It is also recognized that more recently technologies such as Slip-Sweep have been utilized. See, for example, (Postel, et al., 2005) for a summary. It is recognized that in some very large scale 3-D Vibroseis seismic surveys, there are many individual vibrators that are sweeping in various locations utilizing sweep start times that are random relative to other vibrators. In particular it is recognized that in such surveys there are many vibrators that are individually deployed and physically separated by distances much larger than seismic wavelengths.
It is known in the art that Linear Superposition Source Array Theory may be used whenever there are two or more seismic sources deployed simultaneously at distances closer than a few seismic wavelengths.
The concept of mutual admittance for vertical forces, such as a conventional seismic vibrator unit on a half space, such as the earth's subsurface, is known in the art. Miller and Pursey (1955) disclosed analysis that showed that the partition of energy between compressional body waves, shear body waves, and Rayleigh waves varied dramatically depending on the spacing between vibrators. This was due to mutual admittance effects, and includes significant variations in energy partition beyond that predicted by linear superposition array theory. The Miller and Pursey (1954, 1955) analysis is idealized in that it is for a half-space model of the earth, for a homogeneous and isotropic earth, does not include any attenuation, and is for linear elasticity. This type analysis is still a topic of current research; see, e.g., Sanchez-Sesma (2011).
Cassand and Lavergne (1971) describe mutual admittance effects specifically in the context of vibroseis seismic surveys. For example, their FIG. 88 shows the variation of total energy, summed over all azimuths around the center of a vibrator array comprised of three vibrators deployed in an equilateral triangle, for each of the wave types: compressional body, shear body and Rayleigh surface waves (ground roll). They describe, particularly, that the amplitude of the Rayleigh wave can vary significantly as a function of the vibrator spacing.
U.S. Pat. No. 5,111,040 (“Method for Calculating the Optimum Vibrator Spacing for Ground Roll Reduction”, inventor D. Nyland, granted Apr. 23, 1996) discloses a specific technique to optimize vibrator spacing, utilizing the mutual admittance concepts such as analyzed mathematically in Miller and Pursey (1955). The '040 patent discloses techniques requiring a portion of a circular-arc of receivers. The method disclosed in the '040 patent yields results showing total energy for the combination of compressional body waves, shear body waves, and ground roll (Rayleigh) waves with respect to vibrator separation. The total energy is for all wave types inseparably summed together. The total energy includes both linear superposition source array effects and mutual admittance effects, inseparably summed together.
U.S. Patent Application Publication No. 2011/0272207 A1 (“Method of Acquiring Vibroseismic Data Concerning a Zone of the Subsoil, and Seismic Exploration Method Including Such a Method”, Inventor J. Meunier, Pub, Nov. 10, 2011) and International Application Publication WO 2010/079236 A1 (“Method of Acquiring Vibroseismic Data Concerning a Zone of the Subsoil, and Seismic Exploration Method Including Such a Method”. Inventor J. Meunier, Pub, 15 Jul. 2010) disclose a method to reduce the effect of ground roll (Rayleigh waves) by using time shifts of seismic waves that vary non-linearly with variations of the amplitude of the vibrator ground force. Martin and Jack (1990) earlier described some of these type time shifts for body waves, using measurements in wells. It is known in the art that this technique to separate ground roll from other seismic data apparently depends on non-linear elasticity phenomena, and does not utilize mutual admittance effects.
The use of vibrator arrays with all vibrators in-line does not offer any linear superposition source array effect in the cross-line direction, orthogonal to the linear source array.